Paint manufacturers normally formulate coatings to firmly adhere to the surface of objects to which the paint is applied. Understandably, as paint formulators have become more successful in developing durable paints, the efficient removal of those paints has become a more burdensome task. Traditionally, scraping, burning and dissolving have been used with varying degrees of success to remove paint. Where the paint strongly adheres to the object surface, scraping requires forceful direct contact with the entire surface of the coated object. Indeed, several scraping passes may be necessary to completely remove paint from a surface, and the object itself may become undesirably worn, scarred or damaged during the scraping process. Burning, of course, is limited to cases where the object is itself not combustible, and generally requires a substantial investment of time to isolate the object in a combustion chamber, to heat the object sufficiently to oxidize the paint, and to cool the object for removal from the chamber. Moreover, fuel use may be substantial, and some coatings may even leave a residue on the object surface. Solvent treatment often requires costly and hazardous chemicals which must be disposed of or reclaimed after use. Care must also be taken to remove residual solvent from the treated object.
A less traditional method of removing coatings involves substantial cooling of the coated object. The difference between the expansion coefficients of the coating material and the underlying object causes a stress during cooling which reduces the adhesion of the paint to the object. Some paints become brittle and crack. An early use of cooling during the removal of a paraffin coating for motion film is disclosed in U.S. Pat. No. 2,854,360 to Pajes.
As refrigerants such as dry ice, liquified air and liquid nitrogen have become more affordable and available, their use in various commercial operations has expanded. For example, U.S. Pat. No. 3,741,804 to Stapleton describes immersion of machine parts in liquid nitrogen so that a protective layer of ice may later be formed thereon by condensation from a moist environment. Surface removal by spraying with solid dry ice particles is described in U.S. Pat. No. 3,702,519 to Rice et al. Cooling has also been used in such diverse processes as conditioning leather goods (see U.S. Pat. No. 3,130,083 to Turner) and cleaning dishes (see U.S. Pat. No. 3,419,427 to Plock).
Precooling has been used to embrittle surface layers to facilitate their removal. U.S. Pat. No. 3,527,414 to Schorsch shows a process for striking insulation from wire by enbrittling the insulation and thereafter grinding the wire until the insulation is fragmented and removed. U.S. Pat. No. 4,312,156 to McWhorter describes a housing in which articles may be cryogenically cooled and then subjected to shot blasting under cryogenic conditions for surface alteration. A somewhat similar process is described in U.S. Pat. No. 3,934,379 to Braton et al. for removing organic coatings from articles. These operations must be carried out in a cryogenic environment and thus require fairly sophisticated machinery including refrigerated chambers.
In addition to the grinding and blasting shown in the art presented above, processes have used various impact or stress devices for coating removal, such as pressing and cooling (see U.S. Pat. No. 3,845,895 to Nakahara), scoring and breaking (U.S. Pat. No. 4,371,103 to Siemens et al.), plunger striking (U.S. Pat. No. 628,069 to Brown), high-speed needle puncture (U.S. Pat. No. 3,490,664 to Boultinghouse), and hammer striking (U.S. Pat. No. 543,771 to Kennedy). A more modern impact method uses a hand-held pneumatic needle scaler which reciprocates a cluster of chisel needles. As the chisel needles reciprocate, the user presses the chisel needle ends against a surface of an article to descale, strip, or clean that surface. The needles are sufficiently flexible to conform to and scrape a wide variety of surface conformations. Nevertheless, the surface treatment can be time-consuming when the scale, paint, or dirt adheres tightly to the surface of the object.